BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields

Abstract

By using a dc-slice imaging technique, photodissociation of 1,2-C₂H₄BrCl was investigated at 800 nm looking for heteronuclear unimolecular ion elimination of BrCl⁺ in an 80 fs laser field. The occurrence of fragment ion BrCl⁺ in the mass spectrum verified the existence of a unimolecular decomposition channel of BrCl⁺ in this experiment. The relative quantum yield of the BrCl⁺ channel was measured to be 0.8%. By processing and analyzing the velocity and angular distributions obtained from the corresponding sliced images of BrCl⁺ and its partner ion C₂H₄ ⁺, we concluded that BrCl⁺ came from Coulomb explosion of the 1,2-bromochloroethane dication 1,2-C₂H₄BrCl²⁺. With the aid of quantum chemical calculations at the M06-2X/def2-TZVP level, the potential energy surface for BrCl⁺ detachment from 1,2-C₂H₄BrCl²⁺ has been examined in detail. According to the ab initio calculations, two transition state structures tended to correlate with the reactant 1,2-C₂H₄BrCl²⁺ and the products BrCl⁺ + C₂H₄ ⁺. In this entire dissociation process, the C-Br and C-Cl bond lengths were observed to elongate asymmetrically, that is, the C-Br chemical bond broke firstly, and subsequently a new Br-Cl chemical bond started to emerge while the C-Cl bond continued to exist for a while. Hence, an asynchronous concerted elimination mechanism was favored for BrCl⁺ detachment.